The crossword solution for the term CURVE, AS THE BACK is ARCH, which is a four-letter word. An ARCH is a curved structure that is used to span an opening and support the weight above it. Arches are commonly used in architecture, such as in doorways, windows, and bridges. They are also used in engineering, such as in the construction of dams and aqueducts. Arches are often used to create a decorative effect, as well as to provide structural support.